NASA's Voyager Probes to Leave Solar System by 2016

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It may be decades before humanity sets foot on Mars, but we're
only five years away from sampling the vast stretches of
interstellar space beyond our solar system for the first time,
researchers say.

NASA's twin unmanned
Voyager spacecraft, which were launched in 1977, are
streaking toward the edge of the solar system at around 37,000
mph (60,000 kph). At that rate, they'll probably pop out of our
sun's sphere of influence and into interstellar space by 2016 or
so, according to mission scientists.

"They are about to break free of the solar system," Ed Stone,
Voyager project scientist at Caltech in Pasadena, Calif., said
during a media teleconference yesterday (April 28). "We are
trying to get outside of our bubble, into interstellar space, to
directly measure what is there."

A long history of exploration

Voyager 2 was launched on Aug. 20, 1977, and its twin Voyager 1
blasted off a few weeks later, on Sept 5. Both spacecraft were
tasked mainly with studying Jupiter, Saturn and their moons.
[ 5
Facts About NASA's Voyager Spacecraft ]

The spacecraft are also carrying so-called "golden records"
containing the distilled essences of humanity, such as various
musical offerings and greetings to the universe in 55 different
languages. The goal is to teach
alien civilizations a little about us, should they ever pluck
the Voyagers out of the void.

In their early years, the Voyagers made a series of important
discoveries about the giant planets. For example, the mission
detected active volcanoes on Jupiter's moon Io – the first time
such features were found beyond Earth. The spacecraft also found
evidence of a liquid-water ocean beneath the icy surface of
Jupiter's moon Europa.

"Each of these discoveries changed the way we thought of other
worlds," Stone said in a statement.

The Voyagers made it past Saturn, with the spacecraft examining
Neptune and Uranus as well. And then they just kept on going,
zooming toward the edge of the solar system in different
directions and different planes.

Voyager 1 is now about 11 billion miles (17.7 billion kilometers)
from Earth, while Voyager 2 is about 9 billion miles (14.5
billion km) away, Stone said. Voyager 1 is the most far-flung
human-made object in the universe. [ NASA's
10 Greatest Science Missions ]

Probing the heliosheath

While the Voyagers have left the planets well behind, they're not
beyond the solar system yet. They're still within a huge bubble
called the heliosphere, which is made of solar plasma and solar
magnetic fields. This gigantic structure is about three times
wider than the orbit of Pluto, researchers said.

Specifically, the Voyagers are plying the heliosphere's outer
shell, a turbulent region called the heliosheath.

"We're smelling, we're touching the ionized matter in the
heliosheath," said Merav Opher of Boston University, a Voyager
guest investigator.

The Voyagers are helping scientists better understand the
mysterious heliosphere. For example, measurements from the
spacecraft revealed that the structure is distorted and
asymmetric, yanked out of shape by the interstellar magnetic
field, researchers said.

And in June 2010, Voyager 1 measured the outward velocity of the
solar wind -- the million-mile-per-hour stream of charged
particles coming from the sun -- to be zero in its location in
the heliosheath. That surprising reading hasn't changed since.

Researchers don’t think the solar wind has stopped out there;
they believe it may have just turned a corner. So they've
recently started ordering Voyager 1 to do a
series of acrobatic maneuvers, to point its instruments in
different directions so the craft can pick up and track the
puzzling solar breeze.

Breaking free

The heliosheath looks to be about 3 to 4 billion miles (4.8 to
6.4 billion km) thick, and the spacecraft are already well into
it. Based on their speed, they should be out in about five years,
Stone said.

That time frame is manageable. The Voyagers' radioisotope
thermoelectric generators -- which convert the heat emitted by
plutonium's radioactive decay into electricity -- can power their
instruments until at least 2020. And the spacecraft have enough
hydrazine fuel left to perform maneuvers for another 60 years,
researchers said.

Of course, there are no signposts marking the start of
interstellar space, where the Voyagers will escape the sun's wind
and magnetic field only to be buffeted by those of other,
far-flung stars. So astronomers will probably have a hard time
knowing when the historic moment occurs.

"We are starting to talk about what we expect to see," Stone
said. "I suspect, like in the past, we will be surprised, and we
may in fact have a debate for a year or two before we finally
decide, 'We have crossed the boundary.'"

You can follow SPACE.com senior writer Mike Wall on
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